METHODS OF OBSERVATION 



319 



is horizontal, so that the point A is above. Geotropic 

 stimulation is at once followed by a responsive current 

 which flows through the galvanometer from N to A, the 

 upper side of the organ then exhibiting excitatory reaction 

 of galvanometric negativity (right-hand illustration, fig. 193). 

 When the peduncle is brought back to the vertical position, 

 geotropic stimulation disappears, and with it the responsive 



current. 



Experiment 176. Electric variation on the lower side. — 

 The peduncle is now displaced through — 90°, so that the 



Fig. 193. Diagrammatic representation of geo-electric response. 



The middle figure represents vertical position. In figure to the 

 right, rotation through + 90° has placed a above with 

 induced electric change of galvanometric negativity at a. 

 In the figure to the left, rotation is through — 90°, a being 

 below. The electric response is induced galvanometric 

 positivity at a. For simplification of the diagram, the 

 vertical position of the sepal n is not always shown. 



point A, which under rotation through + 90° faced 

 upwards, is now made to face downwards. The direction 

 of the current of response is now found to have undergone 

 a reversal ; it now flows from A on the low^r side to the 

 indifferent point N ; thus under geotropic action the lower 

 side of the organ exhibits galvanometric positivity (left- 

 hand illustration, fig. 193). 



It is obvious that when electric connections are made 

 on tw^o diametrically opposite sides A and B of the shoot, 

 inclination of the organ through 90° to the vertical makes 

 the upper side A galvanometrically negative whilst the 

 lower side B is rendered galvanometrically positive. The 

 resulting electromotive variation is therefore additive. Such 



